L8: HVAC Flashcards
What does HVAC stand for?
Heating and humidification
Ventilation
Air conditioning and dehumidification
Is HVAC required?
Heating - yes
Cooling - no
What does HVAC achieve?
Temperature control
Humidity control
Air movement
Air filtration
What are the three categories that determine comfort?
Personal/physical
Measurable environment (RH, temp)
Psychological (air movement, noise)
What is relative humidity?
The ratio of moisture in the air compared with the maximum (100% means air cannot contain any more moisture at that temp - higher temp = more moisture)
What is the comfort envelope temperature and relative humidity?
Temperature 68-79ºF
RH targeted at 50%
What other factor is considered in the comfort envelope?
Clothing coefficient
What are the two thermal loading types?
Skin load dominated (outside - 55%)
Internal load dominated (outside - 20%; energy from people/lights/equipment dominate)
What are the three different sources of heat flow/gain through a building envelope?
Temperature difference
Solar radiation
Air infiltration
What is the R value?
Resistance to heat exchange = 1/k (1/conductivity)
What is the U value?
Thermal transmittance = 1/R1+R2+R3+…
What is the thermal gradient?
The gradual temperature change through the materials in the building envelope
How is the heat flow through building envelope calculated?
As the hourly conductive heat loss through the envelope based on area, temperature difference and U value
How is heat fain due to solar radiation calculated?
With complex calculations depending on type, shading, sunlight
How does angle of incidence affect light reflected?
Shallower angle = higher % reflected
What is heat flow by infiltration and what are the three most common causes?
Accidental influx of outdoor air
Floors, walls and ceilings, ducts, fireplaces
What are zones?
Area divisions within a building that operate separately
What are zones commonly based on?
Thermal loads - perimeter (changes with weather) vs core (always cold)
Use of space - tenants, different aromas, different operating hours
Quality of space - high quality gives independent control eg. hotel room, labs
What are the 4 types of HVAC systems?
All-air
All-water
Air-water
Packaged
What are the 4 types of all-air HVAC systems?
Single duct, constant air volume
Multi-zone, constant air volume
Single duct, variable air volume
Double (dual) duct
What is single-duct CAV?
Constant temperature (cooled/chilled by water from chiller/boiler)
Low-velocity duct system
Master thermostat sets temp for whole building
What is multi-zone CAV?
Allows separate temperature control (cannot change between hot/cold mode but can change how hot/cold with device that lets in small amounts of heat/coolth to adjust temp)
Separate ducts from AHU
What is single-duct VAV?
Dampers at terminal outlets adjust air flow
More common/efficient than CAV
What is double-duct?
Separate ducts for hot and cold air go to a mixing box where it is blended to reach the desired temperature providing unmatched comfort control
High velocity system to reduce duct size (less space)
What is an all-water system and what are the two types?
Pipes carry hot/cold water to fan coil units; take air from room, heat/cool, replace
2 pipe and 4 pipe
What is a 2 pipe system?
One supply and one exit pipe; whole building is either in heating or cooling mode
What is a 4 pipe system?
Two supply and two exit (one hot, one cold); more variance as each fan coil can either be in heating or cooling mode irrespective of the others
What do all water systems require and why?
Separate ventilation systems to change air because air is not being changed (just heated/cooled and replaced)
What are air-water systems?
Air gets heated/cooled at fan room and then further heated/cooled when mixing with room air by fan coil unit
What are packaged systems?
Self-contained systems for smaller scale buildings (rooftop units)
Split units - outside compressor and condenser, inside coils and fan
What are 8 components in an HVAC system?
Chilled water plant (evaporator/chiller, compressor, condenser)
Cooling tower
Air handling unit (supply fan, air filters)
Boiler
Ducts
Diffusers/registers
Grills
What do the components in the chilled water plant do?
Evaporator/chiller - liquid expands, cools, becomes vapour (Boyle’s Law) and cools water for AHU
Compressor - increases pressure
Condenser - cool condensing water turns vapour to liquid (warms condensing water)
What does the cooling tower do?
Warmed water from condenser sprayed through air where it cools and returns to the condenser
What does the air handling unit do in cooling/heating mode?
Cooling mode: Unconditioned air (used/outside) enters --> filtered --> passes over cooling coil (with chilled water from evaporator) and condensation drained --> conditioned air (lower temp and humidity)
Heating mode:
Unconditioned air –> filtered –> preheated –> passes over cooling coil (turned off) –> reheated –> conditioned air through ductwork –> humidifier sprays mist to add moisture if necessary
What does the supply fan do and what are the two types?
Provides moderate air pressure to blow air through ductwork
Serial (propeller)
Centrifugal (offset and easier to access)
What do air filters do and what are the three types?
Remove odours, irritants and pollutants
Arrestance - gauze/mesh traps particles; needs regular changing and can reduce air flow when clogged
Washing - incoming air sprayed so particles wash into collection tray; increases humidity and can cause bacteria
Electrostatic - particles magnetically drawn out; most effective
What does the boiler do?
Produces hot water/steam used in air handling unit
What are ducts typically made of and the two classifications?
Sheet metal
Low-velocity or high-velocity (>10ms-1)
What do diffusers and registers do?
Supply air to spaces; diffusers have angled slates to distribute in various directions and registers have a grill and blades so supply air can be controlled
What are grills?
An opening for air to return to the system
How can ductwork be placed?
Vertically (core, structure, edges)
Horizontally (circulation paths, structure edges, plenum)
What is the plenum and how does HVAC work there?
Space between ceiling and above floor deck
No ductwork; air pressurises in plenum space and displaced through openings in the ceiling
What is typical cooling capacity and size of boiler/chiller room, cooling tower and fan room for a 15,000m2 high-rise?
Cooling capacity - 450tons
Boiler/chiller room - 300m2
Cooling tower - 60m2
Fan room - reaches max 25 floors
What is heat flow measured in?
British Thermal Unit / hour or Watts
